Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application No. 102 01 639.9 filed Jan. 17, 2002.
1. Field of the Invention
The present invention relates to a method for the finishing treatment of work pieces with the use of a processing device including a processing tool and a numerically-controlled (NC) device for feeding (or advancing) the tool. The force of the contact pressure applied by the tool for processing or working the work piece is measured on the side of the work piece and the values measured for the force are supplied to the numerical control (NC) that determines the rate at which the processing tool is advanced against the work piece. The finishing treatment is carried out in a force-controlled operating mode in which the feeding rate is controlled depending on the force of contact pressure acting on the working tool.
2. The Prior Art
In the finishing treatment of work pieces, it is necessary to control the movements of the rotating working tool with high precision in order to satisfy high requirements with respect to the accuracy to size and the surface quality of the surface of a work piece being processed. The feed has to be controlled so that the processing tool rests with a defined force against the surface of the work piece to be treated. In the normal case, this force amounts to less than 100 Newtons (N). In the finishing treatment of very small work pieces, forces of contact pressure in the order of magnitude of from 1 to 50 N have to be specifically adjusted with only minimal deviations from the norm.
A method characterized by the features described above is known from DE 199 52 805 A1. In this known method, the feeding device employed is a carriage with a numerically controlled (NC) linear motor that serves as the device feeding the tool. The linear motor is characterized by high dynamics and is controlled depending on the force of the contact pressure acting on the processing tool. The carriage has to be supported free of friction in order to permit precise, force-controlled tool-feeding movements. To accomplish this known method requires extensive technical expenditure.
The object of the invention is to provide a method for the finishing treatment of work pieces with which force-controlled feeding movements of the processing tool are possible with a force of contact pressure that can be adjusted in a delicate and precise manner. Furthermore, the aim is to be able to carry out the method as well with mechanical tool-feeding devices whose setting movements are afflicted with friction.
These and other objects are achieved, in accordance with the invention, by providing a method which employs a device for processing the work piece in which the tool spindle is movably supported on leaf springs on the feeding device and can be driven in the direction of the feeding movement. The processing device includes a device for measuring the force that is effectively working substantially free of deformation and is located between the tool spindle and the feeding device. In the force-controlled operating mode, feed rate values are allocated to the force-measuring system at a cycle control frequency that is predetermined by the numerical control (NC).
The allocation between the values for the force and the feed rate is fixed according to a damping function so that the feeding rate increases with a drop in the measured value of the force, and decreases with a rise of the value of the measured force. This process continues until the value of the measured force corresponds within permissible tolerances with a preset value.
According to the invention, the tool spindle is supported on leaf springs that are aligned transversely in relation to the direction of the feed. The leaf springs, which are arranged vertically and clamped with a short free length, are highly elastic and flexible in the direction of the feed, so that forces of contact pressure applied to the tool spindle are transmitted to the force-measuring system delicately and practically free of loss. In particular when these leaf springs are clamped with a short free length, they are capable of absorbing high vertical forces as well as high transverse forces. The leaf springs are therefore suited for alone supporting the tool spindle on the feeding device without requiring any additional guide between the tool spindle and the tool-feeding device that is afflicted with friction. A carriage driven by a roller spindle can be employed as the feeding device. Frictions occurring in the carriage guide as well as between the roller spindle and the carriage have no effect on the measurement of the force of the contact pressure at which the processing tool is applied against the surface of the work piece. The method as defined by the invention permits a highly sensitive control of the force of the finishing treatment process. Forces of contact pressure are reproducible down to 1 N and can be adjusted with precision and can also be maintained throughout the course of the finishing treatment process in the force-controlled operating mode.
The force-measuring device employed is preferably a piezo element, i.e. a piezoelectric element, that is clamped between a connecting element secured on the tool spindle, and a bearing block arranged on the feeding device. This piezo electric element transmits forces of contact pressure acting on the tool spindle to the feeding device substantially without any deformation.
Suited for the method as defined by the invention are also feeding devices capable of performing multi-axes feeding movements. These devices include a number of feeding drives corresponding with the number of axes. The feed drives associated with the axes are controlled by the numerical control (NC) so that the superposed feed in the direction in which the force of the tool spindle is acting takes place according to the force-controlled mode described above.
According to a further embodiment, the tool spindle for the finishing treatment of a new work piece is advanced in the high-speed mode until a first (or starting) cut of the work piece by the processing tool is recognized via a measuring signal of the force-measuring device. The feeding movement of the tool spindle is stopped by the numerical control (NC) with the recognition of the first (or starting) cut, and the further finishing operation takes place in the force-controlled operating mode. The nominal value of the force applied for the finishing treatment may be varied according to a time function incrementally in steps or continually. The method as defined by the invention permits the finishing treatment of work pieces with defined forces of contact pressure that are optimally coordinated with the work piece as well as with the working tool, specifically with only negligible minor deviations from the norm. When small-sized work pieces are processed, it is possible to achieve forces of contact pressure down to 1 N. The finishing results obtainable with the method as defined by the invention have a very good geometry and very good surface characteristics. By controlling the force, overloading of the processing tool can be avoided, which is advantageously reflected by low wear of the processing tool.